1. Field of the Invention
The present invention relates to a voltage generator, and more particularly, to a voltage generator having a dynamic resistors feedback control.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a voltage generator according to the prior art. The voltage generator 10 comprises a charge pump 12, a voltage dividing circuit 14, and a comparator 16. The charge pump 12 is used for outputting a high voltage Vout and generating a divided voltage Vdiv of the high voltage Vout by the voltage dividing circuit 14. The comparator 16 outputs an enabling signal EN according to the divided voltage Vdiv and a reference voltage Vref. The voltage dividing circuit 14 and the comparator 16 form a feedback loop. The charge pump 12 outputs the high voltage Vout according to the enabling signal EN. The voltage dividing circuit 14 comprises a first resistor 17, a second resistor 18, and a third resistor 19. A first end of the first resistor 17 is coupled to a first node A. A first end of the second resistor 18 is coupled to a second end of the first resistor 17. A second end of the second resistor 18 is coupled to a second node B. A first end of the third resistor 19 is coupled to the second node B. A second end of the third resistor 19 is coupled to a third node C. The first node A is coupled to an output end of the charge pump 12. The second node B is coupled to a first input end of the comparator 16. The third node C is coupled to a ground end. The voltage of the first node is the high voltage Vout, and the voltage of the second node is the divided voltage Vdiv.
Please refer to FIG. 2. FIG. 2 is a waveform diagram of the high voltage Vout and the divided voltage Vdiv in FIG. 1. The feedback loop formed by the voltage dividing circuit 14 and the comparator 16 can limit the charge pump 12 to output the high voltage Vout close to a target level Vtar. When the high voltage Vout outputted from the charge pump 12 rises to the target level Vtar, the feedback loop can turn off the charge pump 12. The charge pump 12 will be turned on until the high voltage Vout is under the target level Vtar. Thus, the comparator 16 compares the divided voltage Vdiv with an upper level and a lower level of a reference voltage Vref respectively. When the divided voltage Vdiv is over the upper level of the reference voltage Vref, the comparator 16 outputs the enabling signal EN, turning off the charge pump 12. When the divided voltage Vdiv is under the lower level of the reference voltage Vref, the comparator 16 outputs the enabling signal EN, turning on the charge pump 12. Since the comparator 16 has a delay time, the feedback loop will make ripples on the output voltage of the charge pump 12.
In conclusion, a high voltage generator utilizes a voltage dividing circuit and a comparator to form a feedback loop. The feedback loop can control the charge pump so that the charge pump is turned off when the output high voltage rises to a target level. The charge pump is turned on until the high voltage is under the target level. The feedback loop can limit the output voltage of the charge pump, but make ripples in the output voltage. Big ripples may result in the output voltage being unstable and may also generate noise.